PANNA: Bt-Corn Pollen from Iowa Fields Kills Monarch Caterpillars

In August, Iowa State University scientists reported in the journalOecologiathat one kind of Bt-corn pollen naturally deposited on milkweeds in and near corn fields kills monarch butterfly caterpillars. This research confirms a laboratory study published inNaturelast summer which showed that Bt-corn pollen is lethal to monarchs.

The Iowa scientists conducted three kinds of studies using two types of Bt corn marketed by Novartis Seeds: KnockOut, which contains a Bt gene named Event 176 and YieldGard with a Bt gene called Bt 11. KnockOut pollen typically contains more Bt toxin than YieldGard pollen. The three experiments were:

* To learn how much pollen caterpillars might be exposed to under field conditions, researchers counted pollen that fell on milkweeds, the favorite food of monarch caterpillars, within and near KnockOut, YieldGard and nonBt-corn plots.

* To assess mortality of caterpillars exposed to natural, field-deposited pollen, researchers placed caterpillars on pieces of leaves taken from within and at the edges of plots of KnockOut and nonBt-corn and counted the number of dead larvae after two days' feeding.

* To determine the impacts of a range of Bt-pollen densities likely to be encountered in the field, the scientists conducted a laboratory study exposing larvae on pieces of leaves to three levels of KnockOut, YieldGard and nonBt pollen. They counted dead caterpillars and monitored the survivors until they emerged into butterflies, looking for side effects like slowed development time and smaller butterfly bodies.

In the first study, the scientists found that milkweed plants placed in and near both KnockOut and YieldGard corn fields received amounts of Bt pollen that could kill monarch caterpillars. The second experiment revealed that significantly more caterpillars died after feeding for two days on pieces of leaves taken from KnockOut corn fields compared with leaves taken from nonBt fields and leaves with no pollen. In the laboratory study of different pollen densities, caterpillar mortality was significantly greater on the two highest densities of both KnockOut and YieldGard pollen than on nonBt pollen. At the lowest density, larvae survived equally well on Bt and nonBt pollen. Caterpillars that survived exposure to Bt and nonBt pollen appeared to develop similarly into adult butterflies.

The authors suggest that the effects of Bt-corn pollen on monarchs will be greatest where most of the pollen falls--inside Bt-corn fields or within three meters of the edges. Milkweeds are found both within and along the margins of corn fields.

This report follows on the heels of a University of Illinois study published in theProceedings of the National Academy of Sciencesthis summer which suggested that another kind of Bt-corn pollen, Monsanto's Bt corn (Mon 810 gene), was not lethal to swallowtail butterflies under field conditions in Illinois.

What does the Iowa study mean?
This field study does not resolve whether or not Bt corn will be a serious threat to monarch butterflies in corn-growing areas. Much more research needs to be done. But it does mean that the U.S. Environmental Protection Agency (EPA) was remiss in approving Bt corn for commercial use five years ago before fully assessing environmental risks.

EPA is presently engaged in a comprehensive review of all Bt corn and cotton with the aim of deciding within the next year whether and under what conditions Bt crop registrations should be renewed. Current registrations for Bt corn and cotton expire in 2001.

The Union of Concerned Scientists is urging EPA not to reregister any Bt corn until it has a program in place to thoroughly assess ecological risks of Bt crops, including the risk to monarchs and other beneficial insects. In addition, EPA should require farmers to plant buffers of nonBt-corn around Bt fields in the coming year to reduce the amount of toxic pollen blowing beyond transgenic fields.

C.L. Wraight, A.R. Zangerl, M.J. Carroll, and M.R. Berenbaum. 2000. Absence of toxicity of Bt pollen to black swallowtails under field conditions.Proceedings National Academy of Sciences USA, published at PNAS Online June 2000.

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